In recent years, ultrathin fibers called nanofibers with a diameter of less than 1 μm have received attention.
Nanofibers are unique materials that have functions derived from nanoscale diameters and ease of handling derived from macroscale lengths. Typical characteristics of nanofibers include (1) large specific surface area (ultra-specific surface area effect), (2) nanoscale size (nano-size effect), and (3) molecular orientation in fibers (molecular orientation effect).
Many methods for producing nanofibers have been proposed. Mainly known are electrospinning, melt-blow, and island-in-sea melt spinning methods.
The electrospinning method involves application of a high voltage of several tens of kilovolts to a diluted resin solution to scatter the solution by the force of the electric field and to volatilize the solvent simultaneously to form fibers. In this method, the resin should be soluble in any solvent. In addition, the method requires very severe conditions on preparation of the solution and operational parameters, such as temperature, humidity, and electric field. Use of high voltage poses a safety problem in use of organic solvents and a problem on recovery of the solvents. Another disadvantage is low productivity due to use of diluted solutions.
The melt-blow method is a common process for manufacturing non-woven fabrics, which are produced in commercial scales by several companies, for example, Asahi Kasei Corporation and Toray Industries, Inc. Although the melt-blow method is commonly used for producing non-woven fabrics, a reduced volume of resin should be discharged from a nozzle with a small diameter to produce nanofibers by this method. As a result, nanofibers can be produced with significantly low productivity. In addition, usable resins are limited to special grades with low viscosity.
The island-sea melt spinning method involves producing sea-island conjugated yarns each having several tens of to several hundred islands in the sea with a spinet that can dispose a large number of island polymer segments in the sea polymer and removing the sea polymer with a solvent to produce ultrafine fibers consisting of the island polymer. The island-sea melt spinning method has disadvantages, such as high process cost for dissolution of the sea component and low productivity. Furthermore, compatibility between the sea component and the island component having different properties should be carefully considered, and usable resins are limited.
Several methods other than the above-described three methods have been developed. One of them is a carbon dioxide supersonic laser drawing method (Patent Literatures 1 and 2). This method involves irradiating fibers with laser beams in a subsonic to supersonic air stream to partially melt fibers and then drawing the melted fibers by the high-rate air stream.
The carbon dioxide supersonic laser drawing method is characterized in that (1) any thermoplastic polymer material can be applied; (2) the resulting nanofibers have infinite length; (3) the fibers are highly oriented; (4) since no solvent is used, the working environment and the resulting nanofibers are provided with high safeness; (5) since fibers are collected under reduced pressure, nanofibers are not scattered; and (6) since the device has a simple compact structure, it can be placed at any site and is excellent in scalability.
However, the greatest disadvantage of the carbon dioxide supersonic laser drawing method is low productivity, as with other methods for producing nanofibers. Since the air stream derived from vacuum has a limited flow rate, the volume of the resin convertible into nanofibers per unit hour is limited for each nozzle. The material balance between the volume of fed original yarn and the weight of produced nanofibers is represented by the expression: (weight of original yarn)×(feeding rate of original yarn)=weight of nanofibers. According to Examples in Patent Literature 1, the productivity is low, that is, original yarns with a diameter of about 100 to about 200 μm can be fed at a low rate of 0.1 to 1 m/min. The present inventors have also conducted some investigations for further improvement in productivity; however, the productivity was 2.0 m/min at most for original yarns with a diameter of about 100 μm. A higher production rate causes defects such as shot beads and a large diameter of several or larger micrometers of fine fibers, which cannot be named nanofibers. The material balance in volume of original yarn is represented by expression: (radius of original yarn)2×(feeding rate of original yarn)×(radius of nanofiber)2×(air stream rate). In general, fibers that can be referred to as nanofibers are those having a diameter of 500 nm or less. A combination of atmospheric pressure and reduced pressure disclosed in Patent Literature 1 (for example, Examples) can achieve only a limited air stream rate. The expression also demonstrates the limitation of the product, (radius of original yarn)2×(feeding rate of original yarn), i.e., processable volume of original yarn.
Another disadvantage is poor uniformity. As a single fiber can he produced through one orifice in this method, a significantly increased number of nozzles are required for production of a uniform non-woven fabric. However, the number of orifice cannot be readily increased because an increased number of orifices leads to an increase in the number of original yarns, requiring a larger device. In such a circumference, a large number of original yarns cannot be precisely handled. The increased number of original yarns requires complicated designing of the laser optical system and an increased number of laser oscillators. Furthermore, a large number of orifices lead to a large volume of air flow into the device, precluding control of the air flow in the device. The power of the pump should also be enhanced to keep the reduced pressure in the system. Therefore, the number of orifices are not easily increased due to these restrictions.